A chemical milling solution for lithium aluminum alloy is disclosed for manufacturing a surface finish having superior smoothness characteristics. The process is especially useful where material reduction or thinning is required wherein machining would otherwise be impossible or too expensive.
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14. A chemical milling solution consisting essentially of:
between about 8.9 to about 12.0 percent by volume nitric acid; between about 4.7 to about 6.0 percent by volume Chromic Acid; about 1.0 percent by volume Hydrofluoric Acid; and, between about 81.0 to about 85.4 percent by volume water.
1. A chemical milling solution for lithium aluminum alloy comprising:
between about 8.9 to about 12.0 percent by volume nitric acid; between about 4.7 to about 6.0 percent by volume Chromic Acid; about 1.0 percent by volume Hydrofluoric Acid; and, between about 81.0 to about 85.4 percent by volume water.
3. A method of producing a finish on the surface of a lithium aluminum alloy with a roughness height rating of less than 150, comprising:
exposure to a chemical milling solution comprising: between about 8.9 to about 12.0 percent by volume nitric acid; between about 4.7 to about 6.0 percent by volume chromic acid; about 1.0 percent by volume Hydrofluoric Acid; and, between about 81.0 to about 85.4 percent by volume water.
9. A process for chemical milling the surface of a lithium aluminum alloy workpiece comprising the steps of:
exposing said lithium aluminum alloy workpiece to a chemical milling solution which reduces the surface roughness of said workpiece while reducing the mass of said workpiece; monitoring periodically the thickness of said work piece in order to ascertain the extent of surface roughness which has been reduced; and, removing said workpiece from exposure to said chemical milling solution when the roughness of said workpiece has been sufficiently reduced.
8. A process for chemical milling the surface of a lithium aluminum alloy workpiece comPrising the steps of:
exposing said lithium aluminum alloy workpiece to a chemical milling solution which reduces the surface roughness of said workpiece while reducing the mass of said workpiece; monitoring periodically the thickness of said work piece in order to ascertain the extent of workpiece reduction which has taken place; and, removing said workpiece from exposure to said chemical milling solution when the thickness of said workpiece has been sufficiently reduced.
10. A process for chemical milling the surface of a lithium aluminum alloy comprising the steps of:
exposing a lithium aluminum alloy whose composition comprises: about 1.5 to 3.5 percent by weight lithium; about 6.0 percent by weight Magnesium; about 1.0 percent by weight Manganese; and, about 91.5 to 89.5 percent by weight aluminum; to a chemical milling solution whose composition comprises: between about 8.9 to about 12.0 percent by volume nitric acid; between about 4.7 to about 6.0 percent by volume chromic acid; about 1.0 percent by volume Hydrofluoric Acid; and, between about 81.0 to about 85.4 percent by volume water; and, ceasing said exposure of said lithium aluminum alloy to said chemical milling solution.
2. The chemical milling solution of
said nitric acid is 10 percent by volume; said chromic acid is 5 percent by volume; and, said water is 84 percent by volume.
4. The method as recited in
5. The method as recited in
6. The method as recited in
7. The method as recited in
11. The process, as recited in
12. The process, as recited in
13. The process, as recited in
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The government has rights in this invention pursuant to Contract No. F33657-81-C-2108 awarded by the Department of the Air Force.
Lithium aluminum extruded alloys for utilization in the aerospace industry are available with the proper structural orientation and surface characteristics, but wherein the alloy is too thick requiring reduction in weight of the alloy. This is due to the fact that many extruded shapes cannot be manufactured as thin-walled extrusions. Those shapes which, due to their shape, must be extruded with a minimum thickness must subsequently be thinned.
Alloys other than lithium-aluminum have been successfully chemically milled. However, in the case of lithium-aluminum alloy, previous efforts at chemical milling have produced surface finishes which are unacceptable.
Surface roughness of metallic parts can be measured using a "roughness height rating" or RHR scale. RHR is an arithmetical average in microinches of the surface deviations from absolute smoothness. Standards in the aerospace industry dictate that the surface roughness of the lithium aluminum metal have a roughness height rating RHR of from less than 250 to 125.
Standard chemical milling processes in the aerospace industry for lithium aluminum alloy use a solution containing from 10% to 25% NaOH in water along with from 2 to 6 ounces elemental sulphur per gallon of the water - NaOH mixture. The mixture is heated to from 190 to 200 degrees Fahrenheit. This unsuccessful chemical milling mixture will produce a surface having a roughness height rating of not less than 500. Mechanical methods for producing a satisfactory surface roughness after such unsuccessful chemical milling to this reduced roughness standard are cost prohibitive.
The chemical milling solution of the present invention has produced a surface of satisfactory roughness having a roughness height rating of from 40 to 61. The process has been successfully practiced upon alloy 2090-T8ES1, a lithium aluminum extrusion. The process is also useful for reducing the thickness of the metal as desired and performs the reaction at room temperature. Chemical milling practiced using the solution of the present invention requires no heat, and desmutting of the metal surface is not required.
Alloy 2090-T8ES1, upon which the invention has been successfully practiced, has the following composition:
| ______________________________________ |
| Component Composition in alloy |
| ______________________________________ |
| Lithium 1.5-3.5% |
| Magnesium 6.% |
| Manganese 1.% |
| Aluminum 91.5-89.5% |
| ______________________________________ |
The solution used to successfully chemical mill the Lithium Aluminum extrusion described above is as follows:
| ______________________________________ |
| Component Composition in solution |
| ______________________________________ |
| Nitric Acid 8.9% to 12.0% |
| Chromic Acid 4.7% to 6.0% |
| Hydrofluoric Acid |
| approximately 1.0% |
| Water 81.0% to 85.4% |
| ______________________________________ |
EXAMPLE
The surface dimensions of a sample of alloy 2090-T8ES1, a lithium aluminum extrusion, were measured and recorded. The sample was then suspended in a horizontal position within a tank containing the chemical milling solution of the present invention. The roughness of the starting material ranged from an RHR rating of 30 to 60. The lithium aluminum sample was horizontally positioned in a manner to cause the most severe gas entrapment for a worst case test. The sample was an elongate metal member having a flattened plate-like base bisected into a forward and rear portion along its length by a planar surface member attached to the elongate metal member at an angle along the metal member's center. The sample was measured each morning for three days. On the third day the test was concluded. At that time all pre measured points on the sample were again measured and recorded. The following results were obtained:
Immersion time--74.5 hours
Average metal removed per surface--0.028"
Chemical milling rate--0.00039" per hour
Surface roughness--40-61 RHR (cross grain)
| ______________________________________ |
| Thickness readings, before and after chemical milling |
| (sample results in mils at three points in each area) |
| ______________________________________ |
| Base forward |
| Leftmost section |
| Center section |
| Rightmost section |
| Before After Before After Before |
| After |
| ______________________________________ |
| 349 288 347 287 345 290 |
| 345 285 340 285 337 283 |
| 340 283 343 284 342 287 |
| ______________________________________ |
| Base rearward |
| Leftmost section |
| Center section |
| Rightmost section |
| Before After Before After Before |
| After |
| ______________________________________ |
| 342 286 342 285 340 286 |
| 341 286 340 287 341 286 |
| 348 292 345 290 347 292 |
| ______________________________________ |
| Attached member |
| Leftmost section |
| Center section |
| Rightmost section |
| Before After Before After Before |
| After |
| ______________________________________ |
| 245 188 241 187 243 187 |
| 244 184 239 185 241 186 |
| 243 188 241 187 242 188 |
| ______________________________________ |
No other ingredients are required. During the chemical milling, the hydrofluoric acid is consumed in the reaction. Consequently, as the hydrofluoric acid is consumed, the etch rate will decrease. It is recommended to add hydrofluoric acid as needed to maintain a milling rate in the latter stages of chemical milling equivalent to about 50% of the initial milling rate. This step will assume greater or lesser importance depending upon the quantity and composition of the metal to be milled and the volume of solution in contact with the metal.
The foregoing was a description of a process for chemical milling of lithium aluminum alloy. The process results in a metal surface finish with superior smoothness characteristics. The process is also useful for controllably reducing the thickess of the metal. The rate of metal reduction can be readily controlled by controlling the strength of the hydrofluoric acid in the solution. The surface roughness reduction and reduction in thickness can, as recited in the example, be accomplished simultaneously. The chemical milling solution produces metal surfaces having a roughness height rating (RHR) of 40 to 61, and the surface of the metal after chemical milling does not require desmutting.
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| May 13 1988 | BRUCE, LELAND E | MCDONNELL DOUGLAS CORPORATION, A MD CORP | ASSIGNMENT OF ASSIGNORS INTEREST | 004935 | /0792 | |
| May 19 1988 | McDonnell Douglas Corporation | (assignment on the face of the patent) | / |
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